Visual contents, whether fixed or moving pictures, are normally creations that are covered by copyright-related guarantees of exclusivity. Reproducing them is normally allowed only within a strictly defined framework which allows the authors and their beneficiaries to be remunerated.
In order to ensure that these legal rules are correctly observed, numerous systems have been developed to prevent illegal copies or sufficiently degrade their quality to make them unusable.
In this context, the object of patent application EP 1 237 369 is to combat the copying of images by picture taking when they are displayed, for example by a camcorder in a cinema auditorium. To this end, it is proposed to modulate the luminance of the pixels of a pattern around the value to be displayed to a high frequency that makes the pattern invisible to the human eye but that generates artefacts on the sequence filmed by the camcorder. This pattern is commonly called a tattoo or an anti-copy pattern.
The form of the pattern is determined to produce, for example, messages of the “ILLEGAL COPY” type that will appear in the images displayed by the camcorder.
For the pattern to be invisible to the naked eye, the modulation consists in alternating images in which the pattern is light with images in which it is dark, the average intensity of the pattern over several images corresponding to that to be displayed in the images in the absence of a pattern. When these images are displayed, the eye performs an integration and, in fact, perceives the average intensity.
Another method consists in modulating the colour of the pixels of the pattern without modifying their luminance. The colour of the pixels of the pattern is modulated around the colour to be displayed at a high frequency that makes the pattern invisible to the human eye. This method is then based on colour fusion. It is described in very great detail in international patent application WO 05/027529.
Generally, the purpose of this temporal modulation is to distribute over time a parameter that is received at a given instant t. This parameter is normally linked to the video and can be, as indicated previously, the luminance or colour. The temporal distribution is done at instants separated by “frame” or “subframe” times.
This technique does, however, present a problem when the images represent a moving scene. In practice, since the eye tends to follow the movement in the image, the temporal integration is no longer done correctly and the pattern then appears to the naked eye. Consider the example of a modulation creating a luminance deficit for a pixel P of the pattern in a first image and a complementary luminance surplus for the same pixel in a second picture. If the eye does not move, it adds together the luminances of these two pixels and then perceives the average luminance value. The perception of the eye is then correct. If the eye moves, the pixel P in the first image is not integrated by the same retinal area of the eye as this same pixel in the second image. The visual sum between these two pixels is no longer correct and the pattern is then detected by the eye.
To overcome this problem, it is possible to have the pattern move in accordance with the movement of the eye in such a way that the latter integrates the video parameters relating to one and the same pixel in the two successive images displayed. This movement compensation technique can be used to limit the defects and improve the quality of the processing but presupposes a high accuracy of the movement vectors and a good reliability of the movement estimator.